Definition:
Whimsical - erratic in behavior or degree of unpredictability.


Example:
"The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well. Either the well was very deep, or she fell very slowly, for she had plenty of time as she went down to look about her and to wonder what was going to happen next."
-Alice in Wonderland by Lewis Carroll


Explanation:
Alice in Wonderland is a story about a girl that goes to a dimension of strangeness and wonder. Whimsical things happen left and right and therefore the tone is whimsical.